1. Field of the Invention
The present invention relates to a method for producing partially expanded bodies with a PVC-based embryo cellular structure and to a mold for the production of these bodies.
More precisely, the method and the mold which form the subject of the present invention are advantageously used for the production of partially expanded bodies (called xe2x80x9cembryo cellular bodiesxe2x80x9d in the specific technical sector) which, as a result of subsequent chemical-physical treatment, can be used to obtain expanded products, mainly in the form of panels, which can be used as thermal/acoustic insulators in the building sector or for applications in the naval or aeronautic sector or also for imparting floating properties to boats, etc.
1. Description of the Prior Art
As is known, the production process which results in the manufacture of the panels envisages initially the formation of a paste consisting of a mixture of powders (PVC and other compounds) and liquid substances (in particular isocyanates).
Subsequently the paste is subjected to pressure, heating and subsequent cooling inside suitable molds from where the so-called xe2x80x9cembryo cellular bodiesxe2x80x9d are then extracted. The latter, following further heat treatment in water or steam ovens, are further expanded as a result of the hydrolysis reaction of the isocyanate groups present in the material, giving rise to the formation of the panels.
At present, the methods for the production of xe2x80x9cembryo cellular bodiesxe2x80x9d envisage filling the molds with quantities of pastes greater than that required (with an excess amount, in terms of weight, equal to about 8% of the product leaving the mold) so as to allow gaseous inclusions to escape from the molds during the heating step. In other words, the excess paste introduced into the molds allows, at the end of processing, panels with a perfectly regular shape, devoid of gaseous forms or cavities, to be obtained.
In accordance with a first known embodiment of a method and apparatus for the production of xe2x80x9cembryo cellular bodiesxe2x80x9d, the excess paste equivalent to 8% by weight emerges from the top edge of the mold. Generally this occurs after about 5-6 minutes from the start of heating of the molds following the increase in pressure which occurs inside them owing to the chemical reactions which develop in the pastes. Consequently there is a non-recoverable wastage of material (since at high temperature the PVC gelatinizes and the expanding substances deteriorate). At the same time the drawing off of the harmful vapors is necessary, with a consequent obvious increase in the production costs which have a negative effect on the cost of the final product.
More recently, in order to overcome at least partly these drawbacks, a second embodiment of a method and apparatus for the production of xe2x80x9cembryo cellular bodiesxe2x80x9d has been devised, by means of which it is possible to recover and therefore re-use the excess paste introduced initially into the molds, thereby avoiding the wastage of material and the danger of environmental pollution caused by the evaporation of the iscyanates.
This second embodiment, which is described in the patent No. IT 1,242,873 and envisages the extraction of the excess paste during a step of the process involving heating of the paste in the molds, has in practice proved to have certain drawbacks.
In fact, it uses means for extraction of the excess paste which are costly, impractical, pose considerable maintenance problems and do not allow subsequently basically an optimum re-use of the excess paste.
The main object of the present invention is therefore that of overcoming the drawbacks associated with the methods and the molds of the known type by providing a new method for producing partially expanded bodies with a PVC-based embryo cellular structure which allows the recovery in a simple and low-cost manner of substantially all the excess paste introduced initially into the molds, avoiding at the same time wastage of material and risks of environmental pollution caused by the evaporation of the isocyanates.
A further object of the present invention is that of providing a mold for the production of these partially expanded bodies which allows recovery of the excess paste initially introduced inside it.
Another object of the present invention is that of providing a mold for the production of partially expanded bodies with a PVC-based embryo cellular structure, which is simple, reliable and does not require high maintenance costs.